Since the earliest days of powered flight, pilots have been testing the limits of the capabilities of their bodies and of their craft, with resulting injuries, death, and destruction of their craft. Of the stresses placed upon powered flight vehicles and their occupants, those originating in the various forces of acceleration have been prominent as causes of injurious strains in both pilots and craft, causing losses of consciousness and vision by pilots and structural failure of the craft.
Early aircraft were not rigid enough to withstand high forces and had relatively low power, which limited the forces to which the pilots were subjected, for the most part, to those below the threshold of disabling stress. In the 1930's with the development of truly high speed warplanes, then jet engines in the 1940's, and rocket engines in the 1950's, attainable forces of acceleration increased markedly, and the problem became extremely acute.
Acceleration is defined as a rate of change of velocity. It may occur linearly simply due to a change in velocity in a fixed direction or it may occur as a result of a change in direction. If the human body is defined as having three axes, by the conventional definition as used in the aerospace industry, see Fraser, Human Response to Sustained Acceleration, NASA SP-103, Library of Congress Catalog No. 66-60042 (1966), the X-axis is perpendicular to the vertebral column in the antero-posterior or in the forward and backward directions, the Y-axis is perpendicular to the vertebral column in the lateral direction to the right and left; and the Z-axis is parallel to the vertebral column through the approximate center of mass of cross-sections of the body and the head perpendicular to the Z-axis.
"Z" is the vertical axis through the approximate centers of mass of the cross-sections of the body and the head perpendicular to the Z-axis. For the purpose of the discussions in this application the subject is facing directly forward in the +X direction. The -Y direction is to the subject's left and the +Y direction is to the subject's right.
"G" is used to indicate the resultant of body acceleration in gravitational units. Thus +G.sub.z indicates that the heart is displaced caudally downward; +G.sub.x indicates that the heart is displaced toward the back; and +G.sub.y indicates displacement of the heart to the left of the subject, for accelerative displacements respectively upward, forward, and to the right of the subject.
Acceleration is also classified according to the length of time sustained. Abrupt acceleration is defined by Fraser, Human Response to Sustained Acceleration, supra, as .ltoreq.0.2 seconds, brief acceleration as .ltoreq.10 seconds, and prolonged acceleration as greater than 10 seconds.
"To the human engineer, man is a thin flexible sack filled with thirteen gallons of fibrous and gelatinous material, inadequately supported by an articulated bony framework. Surmounting this sack is a bone box filled with gelatinous matter attached to the sack by means of a flexible coupling of bony and fibrous composition. Fuel and lubricants are conveyed to all parts of this machine by flexible hydraulic systems with low pressure tolerances activated by a central pump." Stapp, Military Surgeon, 103:99, 1948.
Thus the primary effect of acceleration is that upon the fluid systems of the body, and particularly on the most vital, the vascular system. A calculation of the effect of a +5G.sub.z acceleration on an upright subject shows that without compensation or adjustment, the blood pressure at the base of the brain would be zero, at the feet equivalent to 370 mm Hg, and the subject unconscious. Physiological compensation may alleviate this somewhat, but this is generally found an accurate prediction.
Acceleration in the -G.sub.z direction is more stressful than in the +G.sub.z direction, with cerebral and petechial hemorrages being observed.
Accleration in the X and Y axial directions has generally been found to have less effect upon the body since it does not place as much stress upon the vascular system. Accordingly, almost all those subjected to high G forces have been in a horizontal or semi-reclining position. This alleviates the gross differentials in hydrostatic pressure, and is also more comfortable in sustained flight.
Special suits have also been widely used, which apply air pressure for breathing and also pressurize the extremities to counteract the hydrostatic imbalance.
Physical and mental training of aircrew are also both highly important in negating and relieving the effects of acceleration. In particular, practice in specialized breathing techniques are helpful.
All of the above techniques are being used to lessen the effects of acceleration, and have their own utility.
One area of study which has been relatively neglected is that relating to time duration. In order to reach an orbital velocity of 18,000 m.p.h., it is necessary to sustain a total acceleration of 820 G-seconds, and to reach an escape velocity, about 1,140 G-seconds. The constraints due to the difficulties in obtaining this prolonged acceleration have made modifications in this area very difficult to achieve, and the approach heretofore has been merely to endure the acceleration and alleviate its effects, rather than attempt to significantly modify it, particularly with positive acceleration to ultimate velocity. Deceleration on re-entry of space vehicles has necessarily been handled by holding maximum forces to those sustainable by the crew.
Several investigators have found that the effects of acceleration are strongly time-dependent, with a latent period of 0.2 seconds required for the development of hydrostatic effects, and some accommodation by the body to longer periods of over 5 seconds.
A thorough discussion of the subject is given by Fraser, Human Response to Sustained Acceleration, supra.
A multi-directional anti-G device is disclosed in U.S. Pat. No. 2,985,413 to Widmanstetter, in which a capsule may be rotated to maintain the occupant with his Z-axis normal to the acceleration. A similar device is also disclosed in U.S. Pat. No. 2,611,562 to Exton. Both of these maintain the body of the pilot with his Z-axis normal to major acceleration but provide no means of neutralizing the movement of body fluids and suspended solids due to the forces perpendicular to the Z-axis. See also T. R. Potter, Acceleration Protection, Bibliography No. 462, North Carolina Science and Technology Research Center, Research Triangle Park, N.C. 27709, Aug. 24, 1976.